SubMICs of penicillin and erythromycin enhance biofilm formation and hydrophobicity of Corynebacterium diphtheriae strains

Comparative evaluation of antimicrobial peptides: effect on formation, metabolic activity and viability of Klebsiella pneumoniae biofilms

Introduction

Klebsiella pneumoniae (K. pneumoniae) is a major human nosocomial infectious agent and an important veterinary pathogen, frequently resistant to various antibiotics. It causes diseases such as pneumonia, urinary tract infections, surgical wound infections and septicemia. Biofilm formation of K. pneumoniae promotes persistent infection and contributes to resistance against antimicrobial agents. The objective of this study was to comparatively evaluate the effect of selected AMPs on the formation, metabolic activity and viability of Klebsiella pneumoniae biofilms of veterinary and human origin.

Methods

Biofilm formation of three K. pneumoniae strains was quantified using the crystal violet assay and visualized by scanning electron microscopy (SEM). The inhibitory effects of eight different AMPs on the formation and metabolic activity of K. pneumoniae biofilms, as well as on planktonic growth, were examined using crystal violet, resazurin and broth microdilution assays, respectively. The effect on living and dead bacteria in mature biofilms was investigated using the fluorescent dyes SYTO™ 9 and propidium iodide. In addition, the distribution of rhodamine B-labeled peptide DJK-5 in mature biofilms of strain 17349 was visualized by confocal laser scanning microscopy (CLSM).

Results

Biofilm formation was confirmed for all three K. pneumoniae strains. Depending on the strain, we found that planktonic growth was affected by the AMPs DJK-5, DJK-6, Onc72, and Onc112. Biofilm formation of all three strains was inhibited by hbD3, LL-37, DJK-5, and DJK-6, with biofilm mass reduced to less than 40% of the untreated control. In addition to the inhibition of biofilm formation, a reduction in the metabolic activity of the biofilm-associated bacteria was also observed. These four AMPs also showed an effect on mature biofilms by reducing the number of both viable and dead bacteria in 22 h-old biofilms. Rhodamine B-labeled DJK-5 took 7 h to visibly accumulate in the planktonic bacteria. Multi-layered biofilm aggregations were mainly negative for rhodamine B-labeled DJK-5, even 44 h after AMP treatment, indicating that certain parts of mature K. pneumoniae biofilms are not accessible for this AMP.

Conclusion

In conclusion, we found differences in the effect of AMPs on biofilms including both increases and decreases in biofilm mass and viability.

Gas Chromatography-mass Spectrometry analysis, Biotoxicity and Antibiofilm Activities of Syzygium aromaticum against Corynebacterium diphtheriae

The objective of this study was to investigate the chemical composition of Syzygium aromaticum essential oil (SAEO), both as its toxicity and biological activities on Corynebacterium diphtheriae. The essential oil (EO) was obtained by hydrodistillation and verified by GC-MS. The main chemical components were eugenol (48.79%), caryophyllene (44.29%) and α-humulene (4.87%). In vitro tests with human red blood cells, blood compatibility. SAEO showed high toxicity for Artemia salina and the bioassay on Tenebrio molitor larvae revealed an average survival rate of 80.00%. The antioxidant activity of SAEO was moderate (40.60%) using the phosphomolybdenum method. SAEO inhibited the growth of all C. diphtheriae strains tested at minimum inhibitory concentrations of 62.5-1000 µg mL-1 (strong to moderate). The minimum bactericidal concentration of SAEO was observed in two strains. Microorganisms cultivated in the presence of subinhibitory concentrations (subMIC) do not show morphological changes. The subMIC of SAEO inhibited in some strains, but in most cases, increased biofilm formation. In conclusion, the results of this study found that SAEO has an inhibitory effect against C. diphtheriae, representing an alternative antimicrobial therapy, although more in vivo biological studies, alone or in combination with occasional antimicrobials, should be carried out to better evaluate its effects.

Porphyrin Production by Corynebacterium diphtheriae Strains from Clinical Isolates

Porphyrins are intermediate metabolites involved in the biosynthesis of vital molecules, including heme, cobalamin, and chlorophyll. Bacterial porphyrins are known to be proinflammatory and are associated with biofilm formation. This study investigated porphyrin production by strains of Corynebacterium diphtheriae using emission spectroscopy, high-performance liquid chromatography with fluorescence detection, diode array detector, and mass spectrometry. Emission spectroscopy revealed characteristic porphyrin emission spectra in all strains, with coproporphyrin III predominating. Qualitative analysis by different chromatographic methods identified coproporphyrin III, uroporphyrin I, and protoporphyrin IX in all strains. Quantitative analysis revealed strain-dependent coproporphyrin III production. Further studies are required to investigate the relationship between porphyrin production and the virulence potential of Corynebacterium diphtheriae.

Macrolides at Clinically-Relevant Concentrations May Induce Biofilm Formation in Macrolide-Resistant Staphylococcus aureus

Macrolides inhibit biofilm formation in several Gram-negative, intrinsically-resistant bacterial species. However, the effect of macrolides upon biofilm formation by susceptible Gram-positive bacteria has been much less explored as such concentrations also inhibit cell growth. To circumvent this problem, the effect of macrolides (erythromycin, clarithromycin and azithromycin) at 0.5-2 µg/mL, upon biofilm formation, was explored on macrolide-resistant Staphylococcus aureus isolates, using the crystal violet assay with 96-well plates. Early (4 h) biofilm formation by strains having constitutive target-modification resistance was consistently induced by all macrolides but not in azithromycin-treated cells in longer (8 and 12 h) incubation. In inducible-resistance isolates, early biofilm formation was enhanced by some macrolide treatments, compared to similar cell growth in the absence of antibiotics; but the typical decay of biofilms at longer incubation appeared prematurely in macrolide-treated cultures. Biofilm formation in an efflux-mediated resistant isolate was not affected by macrolides. These results indicate that macrolides induce the formation of biofilm by resistant S. aureus isolates, especially during the early stages. This suggests that the empirical use of macrolides against infections caused by resistant S. aureus strains could not only result in clinical failure but even in the enhancement of biofilms, making further treatment difficult.

Activity of Ethanolic and Supercritical Propolis Extracts in Corynebacterium pseudotuberculosis and Its Associated Biofilm

Corynebacterium pseudotuberculosis is the etiological agent of caseous lymphadenitis in small ruminants, a chronic disease characterized by the development of granulomas in superficial and visceral lymph nodes as well as in several organs. An important characteristic of the infection with this bacterium is the formation of a biofilm and the absence of effective antibiotic therapy against the disease. From this scenario, the objective of this study was to evaluate the susceptibility of C. pseudotuberculosis to conventional antibiotics and to red, green, and brown propolis extracts obtained by the supercritical and ethanolic extraction methods as well as its activity in the bacterial biofilm. The results of the sensitivity test using antibiotics indicated a sensitivity of C. pseudotuberculosis strains to the antimicrobial agents. The ethanolic extract of green propolis and the supercritical red propolis extract showed the best antibacterial activities against planktonic C. pseudotuberculosis. A lower antimicrobial activity of the brown propolis extract was identified. Propolis extracts were effective in interfering with the formation of the C. pseudotuberculosis biofilm but had little activity on the consolidated biofilm. In conclusion, propolis extracts are more effective against C. pseudotuberculosis in the planktonic stage, being able to interfere with the formation of bacterial biofilm. However, the action of propolis extracts in a sessile and structured microbial biofilm is reduced.

Chemotaxis Toward Crude Oil by an Oil-Degrading Pseudomonas aeruginosa 6-1B Strain

The chemotactic properties of an oil-degrading Pseudomonas aeruginosa strain 6-1B, isolated from Daqing Oilfield, China, have been investigated. The strain 6-1B could grow well in crude oil with a specific rhamnolipid biosurfactant production. Furthermore, it exhibits chemotaxis toward various substrates, including glycine, glycerol, glucose, and sucrose. Compared with another oil-degrading strain, T7-2, the strain 6-1B presented a better chemotactic response towards crude oil and its vital component, n-alkenes. Based on the observed distribution of the strain 6-1B cells around the oil droplet in the chemotactic assays, the potential chemotaxis process of bacteria toward crude oil could be summarized in the following steps: searching, moving and consuming.

Influence of Selected Factors on Biofilm Formation by Salmonella enterica Strains

Biofilm formed by S. enterica on the surface of gallstones or biomaterials promotes the development and spread of chronic infection. The aim of the study was to assess biofilm formation on the surface of polystyrene depending on nutritional conditions and the effect of 0.5, 1.0, and 2.0% glucose and 3.0% bile and sub-inhibitory concentrations of ampicillin on biofilm formation of S. enterica. Sixty-nine clinical strains of S. enterica isolated from feces (92.8%) and blood (7.2%) collected from patients (66.7%) and carriers (33.3%) were used in the study. Assessment of forming 24-h biofilm by these strains was performed on the surface of polystyrene 96-well plates at 37 °C. In this study, it was indicated that 1.0% glucose and 3.0% bovine bile inhibit biofilm formation. Biofilm formation was inhibited in all examined sub-MIC of ampicillin. Biofilm formation is varied in different conditions, depending on the serovar.

Separation of phenolics from peony flowers and their inhibitory activities and action mechanism on bacterial biofilm

Separation and enrichment of phenolics from peony flowers were performed to improve the anti-biofilm and antibacterial activities for the first time. Through several times of separation, the purity of phenolics components increased significantly, and the anti-biofilm and antibacterial activities of phenolics components against E. coli and S. aureus were also significantly improved. Finally, the phenolics of peony flowers in the eluent of silica gel column chromatography (PPF-ESGCC) were found to exhibit the highest anti-biofilm and antibacterial activities. The inhibition rates of PPF-ESGCC on biofilms of E. coli and S. aureus were 77.93%, and 87.03% respectively, at a very low concentration (1/2 MIC, 0.235 mg/mL). It was found that the biofilm inhibition was achieved by inhibiting their swimming, swarming, twitching motilities, exopolysaccharide (EPS) production, and quorum sensing (QS). Moreover, there was a positive dose-dependent relationship (r = 0.75 to 1) between the inhibition rates and concentrations of PPF-ESGCC during the critical biofilm-formation stage (1-3 days). Chemical composition analysis showed the PPF-ESGCC comprised of gallic acid, kaempferol-7-O-glucoside, and apigenin-7-O-glucoside. In conclusion, PPF-ESGCC exhibited strong inhibitory effect on biofilm formation and gallic acid, kaempferol-7-O-glucoside, and apigenin-7-O-glucoside might play a crucial role in inhibiting biofilm formation. Meanwhile, this study indicated that PPF-ESGCC, a new natural QS inhibitor and biofilm inhibitor, could be used as a novel intervention strategy to enhance the safety and quality of food.

Diphtheria

Diphtheria is a potentially fatal infection mostly caused by toxigenic Corynebacterium diphtheriae strains and occasionally by toxigenic C. ulcerans and C. pseudotuberculosis strains. Diphtheria is generally an acute respiratory infection, characterized by the formation of a pseudomembrane in the throat, but cutaneous infections are possible. Systemic effects, such as myocarditis and neuropathy, which are associated with increased fatality risk, are due to diphtheria toxin, an exotoxin produced by the pathogen that inhibits protein synthesis and causes cell death. Clinical diagnosis is confirmed by the isolation and identification of the causative Corynebacterium spp., usually by bacterial culture followed by enzymatic and toxin detection tests. Diphtheria can be treated with the timely administration of diphtheria antitoxin and antimicrobial therapy. Although effective vaccines are available, this disease has the potential to re-emerge in countries where the recommended vaccination programmes are not sustained, and increasing proportions of adults are becoming susceptible to diphtheria. Thousands of diphtheria cases are still reported annually from several countries in Asia and Africa, along with many outbreaks. Changes in the epidemiology of diphtheria have been reported worldwide. The prevalence of toxigenic Corynebacterium spp. highlights the need for proper clinical and epidemiological investigations to quickly identify and treat affected individuals, along with public health measures to prevent and contain the spread of this disease.

A Novel Approach to Study the Effect of Ciprofloxacin on Biofilms of Corynebacterium spp. Using Confocal Laser Scanning Microscopy

Non-diphtherial corynebacteria are Gram-positive rods that cause opportunistic infections, what is supported by their ability to produce biofilm on artificial surfaces. In this study, the characteristic of the biofilm produced on vascular and urological catheters was determined using a confocal microscopy for the most frequently involved in infections diphtheroid species. They were represented by the reference strains of Corynebacterium striatum ATCC 6940 and C. amycolatum ATCC 700207. The effect of ciprofloxacin on the biofilm produced by the antibiotic-susceptible C. striatum strain was evaluated using three concentrations of the antimicrobial agent (2 ×, 4 ×, and 6 × the MIC – the Minimum Inhibitory Concentration). The basis for the interpretation of results was the statistical analysis of maximum points readings from the surface comprising a total of 245 areas of the biofilm image under the confocal microscope. It was observed that ciprofloxacin at a concentration equal to 4 × MIC paradoxically caused an enlargement of areas with live bacteria within the biofilm. Biofilm destruction required the application of ciprofloxacin at a concentration higher than 6 × MIC. This suggests that the use of relatively low doses of antimicrobial agents may increase the number of live bacteria within the biofilm, and further facilitate their detachment from the biofilm’s structure thus leading to the spread of bacteria into the bloodstream or to the neighboring tissues. The method of biofilm analysis presented here provides the original and novel approach to the investigation of the diphtheroid biofilms and their interaction with antimicrobial agents.

Sphygmomanometers and thermometers as potential fomites of Staphylococcus haemolyticus: biofilm formation in the presence of antibiotics

BACKGROUND

The association between Staphylococcus haemolyticus and severe nosocomial infections is increasing. However, the extent to which fomites contribute to the dissemination of this pathogen through patients and hospital wards remains unknown.

OBJECTIVES

In the present study, sphygmomanometers and thermometers were evaluated as potential fomites of oxacillin-resistant S. haemolyticus (ORSH). The influence of oxacillin and vancomycin on biofilm formation by ORSH strains isolated from fomites was also investigated.

METHODS

The presence of ORSH on swabs taken from fomite surfaces in a Brazilian hospital was assessed using standard microbiological procedures. Antibiotic susceptibility profiles were determined by the disk diffusion method, and clonal distribution was assessed in pulsed-field gel electrophoresis (PFGE) assays. Minimum inhibitory concentrations (MICs) of oxacillin and vancomycin were evaluated via the broth microdilution method. Polymerase chain reaction (PCR) assays were performed to detect the mecA and icaAD genes. ORSH strains grown in media containing 1/4 MIC of vancomycin or oxacillin were investigated for slime production and biofilm formation on glass, polystyrene and polyurethane catheter surfaces.

FINDINGS

ORSH strains comprising five distinct PFGE types were isolated from sphygmomanometers (n = 5) and a thermometer (n = 1) used in intensive care units and surgical wards. ORSH strains isolated from fomites showed susceptibility to only linezolid and vancomycin and were characterised as multi-drug resistant (MDR). Slime production, biofilm formation and the survival of sessile bacteria differed and were independent of the presence of the icaAD and mecA genes, PFGE type and subtype. Vancomycin and oxacillin did not inhibit biofilm formation by vancomycin-susceptible ORSH strains on abiotic surfaces, including on the catheter surface. Enhanced biofilm formation was observed in some situations. Moreover, a sub-lethal dose of vancomycin induced biofilm formation by an ORSH strain on polystyrene.

MAIN CONCLUSIONS

Sphygmomanometers and thermometers are fomites for the transmission of ORSH. A sub-lethal dose of vancomycin may favor biofilm formation by ORSH on fomites and catheter surfaces.

Antibacterial fatty acids destabilize hydrophobic and multicellular aggregates of biofilm in S. aureus

Present study is based on 20 methicillin-resistant Staphylococcus aureus (MRSA) isolates recovered from different food items. These isolates were identified on the basis of colony morphology, Gram staining and growth on different selective and differential media. Studies on 16S RNA and positive reactions on DNase agar and Prolex Latex Agglutination system confirm it as Staphylococcus aureus. Oxacillin susceptibility testing and PCR with mecA gene-specific primer results showed that these isolates are MRSA-carrying mecA gene that belongs to SCCmecA type IV and also harbor agr type II. Phenotypic study revealed that these isolates adopt biofilm mode of growth after exposure to subinhibitory doses of oxacillin. The biofilm and cell surface hydrophobicity have a strong correlation. It was noticed that affinity to hexadecane (apolar-solvent) of planktonic cells was low, suggesting its hydrophilic character. However, as the cells are exposed to oxacillin, they adopt biofilm mode of life and the affinity to apolar solvent increases, indicating a hydrophobic character. In biofilm consortia, the cells with more hydrophobic surfaces show incomplete septation and produce multicellular aggregates. This is due to reduced expression of atl gene. This was confirmed by real-time PCR studies. Moreover, the planktonic or wild-type phenotypes of these isolates were more tolerant to antibacterial effect of the fatty acids used; that is, cis-2-decanoic acid and cis-9-octadectanoic acid. These fatty acids were more effective against biofilms. After exposure to these fatty acids, established biofilms were dispersed and surviving cells were unable to readopt biofilm mode of life. The planktonic or wild-type phenotypes produce fatty acid-modifying enzyme (FAME) to inactivate the bactericidal activity of fatty acids by esterification to cholesterol. The biofilm indwellers are metabolically inactive and unable to produce FAME; hence, they are vulnerable to antibiofilm effect of cis-2-decanoic acid and cis-9-octadectanoic acid.

Biofilms: Microbial Shelters Against Antibiotics

Biofilms are communities of aggregated bacterial cells embedded in a self-produced extracellular polymeric matrix. Biofilms are recalcitrant to antibiotic treatment and immune defenses and are implicated in many chronic bacterial and fungal infections. In this review, we provide an overview of the contribution of biofilms to persistent infections resistant to antibiotic treatment, the impact of multispecies biofilms on drug resistance and tolerance, and recent advances in the development of antibiofilm agents. Understanding the mechanisms of antibiotic resistance and tolerance in biofilms is essential for developing new preventive and therapeutic strategies and curbing drug resistance.

Effects of Subinhibitory Concentrations of Ceftaroline on Methicillin-Resistant Staphylococcus aureus (MRSA) Biofilms

Ceftaroline (CPT) is a novel cephalosporin with in vitro activity against Staphylococcus aureus. Ceftaroline exhibits a level of binding affinity for PBPs in S. aureus including PBP2a of methicillin-resistant S. aureus (MRSA). The aims of this study were to investigate the morphological, physiological and molecular responses of MRSA clinical strains and MRSA biofilms to sub-MICs (1/4 and 1/16 MIC) of ceftaroline by using transmission, scanning and confocal microscopy. We have also used quantitative Real-Time PCR to study the effect of sub-MICs of ceftaroline on the expression of the staphylococcal icaA, agrA, sarA and sasF genes in MRSA biofilms. In one set of experiments, ceftaroline was able to inhibit biofilm formation in all strains tested at MIC, however, a strain dependent behavior in presence of sub-MICs of ceftaroline was shown. In a second set of experiments, destruction of preformed biofilms by addition of ceftaroline was evaluated. Ceftaroline was able to inhibit biofilm formation at MIC in all strains tested but not at the sub-MICs. Destruction of preformed biofilms was strain dependent because the biofilm formed by a matrix-producing strain was resistant to a challenge with ceftaroline at MIC, whereas in other strains the biofilm was sensitive. At sub-MICs, the impact of ceftaroline on expression of virulence genes was strain-dependent at 1/4 MIC and no correlation between ceftaroline-enhanced biofilm formation and gene regulation was established at 1/16 MIC. Our findings suggest that sub-MICs of ceftaroline enhance bacterial attachment and biofilm formation by some, but not all, MRSA strains and, therefore, stress the importance of maintaining effective bactericidal concentrations of ceftaroline to fight biofilm-MRSA related infections.

Sub-inhibitory concentrations of penicillin G induce biofilm formation by field isolates of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is a Gram-negative bacterium and causative agent of porcine pleuropneumonia. This is a highly contagious disease that causes important economic losses to the swine industry worldwide. Penicillins are extensively used in swine production and these antibiotics are associated with high systemic clearance and low oral bioavailability. This may expose A. pleuropneumoniae to sub-inhibitory concentrations of penicillin G when the antibiotic is administered orally. Our goal was to evaluate the effect of sub-minimum inhibitory concentration (MIC) of penicillin G on the biofilm formation of A. pleuropneumoniae. Biofilm production of 13 field isolates from serotypes 1, 5a, 7 and 15 was tested in the presence of sub-MIC of penicillin G using a polystyrene microtiter plate assay. Using microscopy techniques and enzymatic digestion, biofilm architecture and composition were also characterized after exposure to sub-MIC of penicillin G. Sub-MIC of penicillin G significantly induced biofilm formation of nine isolates. The penicillin G-induced biofilms contained more poly-N-acetyl-D-glucosamine (PGA), extracellular DNA and proteins when compared to control biofilms grown without penicillin G. Additionally, penicillin G-induced biofilms were sensitive to DNase which was not observed with the untreated controls. Furthermore, sub-MIC of penicillin G up-regulated the expression of pgaA, which encodes a protein involved in PGA synthesis, and the genes encoding the envelope-stress sensing two-component regulatory system CpxRA. In conclusion, sub-MICs of penicillin G significantly induce biofilm formation and this is likely the result of a cell envelope stress sensed by the CpxRA system resulting in an increased production of PGA and other matrix components.

Biofilm-related infections: bridging the gap between clinical management and fundamental aspects of recalcitrance toward antibiotics

Surface-associated microbial communities, called biofilms, are present in all environments. Although biofilms play an important positive role in a variety of ecosystems, they also have many negative effects, including biofilm-related infections in medical settings. The ability of pathogenic biofilms to survive in the presence of high concentrations of antibiotics is called "recalcitrance" and is a characteristic property of the biofilm lifestyle, leading to treatment failure and infection recurrence. This review presents our current understanding of the molecular mechanisms of biofilm recalcitrance toward antibiotics and describes how recent progress has improved our capacity to design original and efficient strategies to prevent or eradicate biofilm-related infections.

Biofilm production by multiresistant Corynebacterium striatum associated with nosocomial outbreak

Corynebacterium striatum is a potentially pathogenic microorganism that causes nosocomial outbreaks. However, little is known about its virulence factors that may contribute to healthcare-associated infections (HAIs). We investigated the biofilm production on abiotic surfaces of multidrug-resistant (MDR) and multidrug-susceptible (MDS) strains of C. striatum of pulsed-field gel electrophoresis types I-MDR, II-MDR, III-MDS and IV-MDS isolated during a nosocomial outbreak in Rio de Janeiro, Brazil. The results showed that C. striatum was able to adhere to hydrophilic and hydrophobic abiotic surfaces. The C. striatum 1987/I-MDR strain, predominantly isolated from patients undergoing endotracheal intubation procedures, showed the greatest ability to adhere to all surfaces. C. striatum bound fibrinogen to its surface, which contributed to biofilm formation. Scanning electron microscopy showed the production of mature biofilms on polyurethane catheters by all pulsotypes. In conclusion, biofilm production may contribute to the establishment of HAIs caused by C. striatum.

Quantitative proteomic analysis of sub-MIC erythromycin inhibiting biofilm formation of S. suis in vitro

Streptococcus suis (S. suis) is a swine pathogen and also a zoonotic agent. Biofilms of S. suis may cause persistent infections by the host immune system and antibiotics. Sub-minimal inhibitory concentration (sub-MIC) of erythromycin can inhibit biofilm formation in bacteria. Here, we performed comparative proteomic analyses of cells at two different conditions: sub-MIC erythromycin treated and nontreated cells. Using iTRAQ strategy, we found some novel proteins that involved in biofilm formation. 79 differentially expressed proteins were identified in sub-MIC erythromycin inhibiting planktonic cell when the protein had both a fold-change of more that a ratio >1.2 or <0.8 (p-value <0.05). Several cell surface proteins (such as Primosomal protein N', l-fucose isomerase, and ABC superfamily ATP binding cassette transporter, membrane protein), as well as those involved in Quorum-sensing, were found to be implicated in biofilm formation. Overall, our results indicated that cell surface proteins played an important role in biofilm formation. Quorum-sensing played a crucial role leading to biofilm formation. ABC superfamily ATP binding cassette transporter, membrane protein and comD might act as channels for erythromycin uptake in Quorum-sensing system. Thus, our data analyzed rough regulatory pathways of biofilm formation that might potentially be exploited to deal with biofilm infections of S. suis. This article is part of a Special Issue entitled: Microbial Proteomics.

BIOLOGICAL SIGNIFICANCE

In this study, we identified many proteins involved in cell transport, biological regulation and signal transduction, stress responses and other metabolic processes that were not previously known to be associated with biofilm formation of S. suis and target spot of erythromycin. Therefore, our manuscript represents the most comprehensive analysis of protein profiles of biofilm formation of S. suis inhibited by sub-MIC erythromycin and provides new proteomic information about biofilm formation.